Hillslope Soils and Organic Matter Dynamics within a Native American Agroecosystem on the Colorado Plateau

retention in alluvium-derived soils of headwater ephemeral streams. Zuni farmers of western New Mexico demonstrate knowledge Peterson et al. (2001) create a convincing case for about soil and hydrological processes that link upland watersheds to alluvium-derived soils that have crucial cultural, ecological, and conservation that maintains or restores the storage cahydrological functions. To define how hillslopes contribute to producpacity of headwater streams. Their data show that headtivity of soils derived from alluvium, we studied soil-vegetation-landwater streams play a key role in the physical and biologiform parameters in three headwater drainages on the Zuni Indian cal integrity of downstream waterways because of their Reservation. Analyses along eight summit to toeslope transects show great extent on the landscape and their proximity to that soil properties follow parabolic and linear trends with changes uplands that are source areas for runoff, sediments, and driven by elevation, lithology, and vegetation in the mesa-canyon nutrients. Properly functioning headwater systems repinyon–juniper–Gambel oak (Pinus edulis Engelm.–Juniperus spp.– tain and recycle a large portion of the nutrients, particuQuercus gambelii Nutt.) woodlands. Total organic C, N, and P concenlarly N, from uplands. This retention and recycling cretrations in surface horizons follow negative parabolic trends and are ates the richest soils and highest biological productivity highest on wooded backslopes and lowest on summits and toeslopes. Inorganic N and available P concentrations and total organic to inorin many arid areas. Bull (1997) points out that many ganic N and P ratios increase linearly from summit to toeslope. Taken Native American farmers recognized the value of the together, soil, landform, and vegetation data suggest: (i) summit posisystem and their activities maintained these functions, tions are relatively stable with immobilizing microbial environments; while modern practices often have the opposite effect. (ii) inorganic nutrients increase progressively down steep and erodible Although many authors describe hillslope models that backslopes as inputs of forest litter are mixed with surface soil; (iii) link soil and nutrient-cycling processes from summits influx of mixed sediment and organic materials from backslopes mainto alluvial toeslopes (Ruhe and Walker, 1968; Conacher tains concentrations of inorganic nutrients on footslopes and toesand Dalrymple, 1977; Schimel et al., 1985; Aguilar and lopes. Entrenchment of drainage ways can circumvent these translocaHeil, 1988), the unique terrain of the Colorado Plateau tion processes. Without the influx of organic materials, footslopes and creates lithologically segmented hillslopes different toeslopes may become nutrient-depleted as immobilization becomes the dominant microbial process. The results underscore the imporfrom those described in accepted hillslope models. Our tance of functional connectivity between upland hillslopes and alluobjective was to develop a hillslope model based on vial soils. linked slope morphology, soils, vegetation, and nutrient dynamics on the Zuni Indian Reservation in New Mexico. We focused on hillslopes in headwater drainages S of the oldest agricultural soils in North America whose sandy to loamy alluvial fans have been used by lie within the Colorado Plateau on alluvial fans generations of Native American farmers. This type of farmed by the Zuni of western New Mexico (Damp et farming continues to be an essential component of many al., 2002). Zuni farmers recognize the role of upland Native Southwestern cultures. By storing runoff and hillslopes in their strategy to produce corn (Zea mays sediments, mitigating destructive floods, and producing L.) and other crops in the semiarid environment (Sandor diverse, productive vegetation for livestock and wildlife, et al., 2002). They actively seek fresh deposits of sedialluvium-derived soils of small upland watersheds also ments and organic material for cultivation, and work to serve vital ecological and hydrological functions (Laenhance fluvial processes that link hillslopes to their gasse et al., 1990; Bull, 1997). fields (Cushing, 1920; Norton et al., 2001; Norton et al., 2002). Sustained productivity of Zuni agricultural soils MATERIALS AND METHODS exemplifies the importance of hillslope processes that We analyzed soil, landform, and vegetation distribution in support crucial cultural, ecological, and hydrological three small watersheds above long-term runoff agricultural functions of headwater alluvial fans (Bull, 1997; Pefields on the Zuni Indian Reservation, New Mexico. The reserterson et al., 2001). This ancient agriculture, which is vation lies on the southeastern part of the Colorado Plateau intimately linked to natural landscape processes, creates at 1800 to 2400 m elevation (Fig. 1) and receives an average of 300 mm of precipitation annually, the majority of which an excellent setting for investigating important relationoften comes during thunderstorms in July, August, and Sepships between upland hillslopes and nutrient cycling and tember. Hillslope morphology on the Zuni Indian Reservation, as in much of the Colorado Plateau, is largely a function J.B. Norton, Dep. of Plants, Soils, and Biometeorology, Utah State of horizontal layers of Cretaceous sandstones and shales. At Univ., Logan, UT 84322-4820, currently at: Cooperative Extension, our study sites, relatively indurate, cliff-forming sandstones act Tuolumne County, Sonora, CA 95370; J.A. Sandor, Dep. of Agronas caprocks over more erodible, slope-forming shales, shaping omy, Iowa State Univ., Ames, IA 50011-1010; C.S. White, Dep. of hillslope profiles much like that defined by Conacher and Biology, Univ. of New Mexico, Albuquerque, NM 87131. Received Dalrymple (1977) (Fig. 2). Headwater drainages typically form 31 Jan. 2002. *Corresponding author ([email protected]). Abbreviations: SOM, soil organic matter. Published in Soil Sci. Soc. Am. J. 67:225–234 (2003).